Why wouldn't they have been brazed or silver-soldered on first? Barrel could be sold as a complete unit. Not really any big deal to inlet the barrel with lugs on it already, so that wouldn't have been a problem.
Think of it this way too: nowadays, most of us inlet the barrel, then inlet the tang, then shape the wrist and lock region before finish polishing the barrel or whatever. This typically involves filing the tang once already in the wood. But if the barrels were breached and being sold already blued, the tang would likewise be finished as well and so the wrist and lock region shaping would need be accomplished with more precise chisel work as opposed to more reliance upon rasps and files.
For all we know the vents were already drilled as well, which would explain the variety of fence locations (relative to the rear of the barrel) dependent upon the type of lock used.
Just some random thoughts.